Employing a single cocatalyst to concurrently mediate reactions with electrons and those with holes toward efficient and stable photocatalysts is highly desirable yet challenging. Herein, two-dimensional (2D) in-plane CdS/Ti3C2Tx sheet-onto-sheet heterostructures have been deliberately constructed, in which Ti3C2Tx, a prototype MXene, is demonstrated to act as a Janus cocatalyst for simultaneously promoting electron-driven photo reduction reaction and inhibiting hole-induced photocorrosion of semiconductor CdS. Ti3C2Tx serves as an electron mediator to boost the electron extraction from CdS for photoreduction reactions. The interlayer assembly maintains the accessible surface reaction sites of CdS, while concomitantly the Cd2+ confinement effect of Ti3C2Tx retards the oxidative holes induced photocorrosion of CdS through preventing Cd2+ leaching. As a result, the photoactivity and stability of CdS are concurrently enhanced through coupling with Ti3C2Tx as a Janus cocatalyst. This facile and double-gain strategy is expected to present a conceptual approach to the rational design of efficient and stable photocatalysts toward solar energy conversion.